Two-phase cooling systems used in space-based operations may employ parallel condensation tubes in a radiator for removing heat from the cooling system. A working fluid in the cooling system, such as ammonia, efficiently absorbs heat, is vaporized, and is then circulated through the radiator for heat removal (condensation). To make the most efficient use of the available heat rejection area of the radiator, it is generally desirable to have the working fluid flow uniformly through the parallel condensation tubes of the radiator. As the working fluid flows through the condensation tubes, the absorbed heat is removed, so that condensed fluid may be pumped back to the heat generating source.
In space-based cooling systems, subcooling of the working fluid, i.e., cooling below its saturation temperature (boiling point), is normally required for practical operation of the working fluid pump. While a gravity feed pump for the working fluid at saturation temperature may be possible in an earth-based cooling system, gravity feed is not possible for space-based cooling systems.
Non-uniform working fluid flow through the parallel condensation tubes of a radiator varies the amount of subcooling provided by each radiator tube. The tubes with higher flow rates will have warmer fluid at their exits. Additional subcooling must be obtained to make up for any lack of local subcooling caused by the non-uniform flow. Additional subcooling obtained from other condensation tubes lowers the average radiator temperature, and thus decreases the radiator heat rejection capability.
In current practice, two techniques are generally used to equalize the working fluid flow through the radiator condensation tubes. These techniques result generally in a relatively lower pressure drop in both the input and output manifolds compared with the pressure drop across the condensation tubes. In this way, these techniques bias the working fluid flow through each of the condensation tubes towards equalization.
According to one technique, the internal diameter of the working fluid input and output manifolds separating the parallel condensation tubes is increased. This technique lowers the pressure drop over the length of the manifolds, so that the manifold pressure drop is small in comparison to the pressure drop across the condensation tubes.
According to a second technique, the pressure drop across the condensation tubes may be increased by decreasing the tube size or by placing an orifice at the condensation tube outlet. This technique also makes the input manifold pressure drop small in comparison to the condensation tube pressure drop.
In space-based cooling systems, the manifolds may be in the range of seventy feet long and the condensation tubes in the range of twelve feet long. The difficulties involved in placing large cooling system equipment in space severely limit weight and volume allowances including the available internal diameter of the manifolds. Thus, normal specifications for a space-based cooling system eliminate the first technique commonly used to improve flow distribution.
Energy limitations in a space-based cooling system also restrict the power available for the pump that may be used in the system to overcome a pressure drop. The total pressure drop available for use over the length of the condenser tubes is therefore limited. Thus, space-based system requirements also practically eliminate the second flow distribution option of increasing the pressure drop across the condensation tubes.
Various inventors have considered some aspects of these problems. U.S. Pat. No. 4,899,810 to J. E. Fredley discloses a low pressure drop condenser/heat exchanger which contemplates elimination of a mechanical pump for inducing working fluid flow. The two-phase system is designed for operation in a micro-gravity environment and includes a capillary pumped loop with a wicked evaporator that produces a working fluid vapor head of about 1/2 PSI upon absorbing heat from a heat source. A heat exchanger for receiving working fluid vapor from the wicked evaporator includes a manifold to direct the vapor to a plurality of parallel fluid channels helically wound about and thermally coupled with a heat pipe that attaches to a radiator panel. If this system were to use a large number of similar heat exchangers, spaced in parallel along a substantially long, small diameter manifold, some mechanism would likely be necessary to distribute the vapor flow evenly between the heat exchangers.
U.S. Pat. No. 5,139,083 to M. L. Larinoff discloses an air cooled vacuum steam condenser with flow-equalized mini-bundles. This device effectively demonstrates a variation of the technique of using an orifice at the tube outlet. A single-row, two-pass steam condensing bundle is used for condensing steam in air-cooled vacuum steam condensers. Each mini-bundle set has one centrally located second pass tube with symmetrically disposed first pass tubes positioned on either side. The steam leaving each first pass tube is controlled by a flow equalizing device installed at the end of the tubes. The flow of gas mixture leaving each second pass tube is controlled by an individual orifice in the gas piping system.
U.S. Pat. No. 4,945,010 to Kaufman et al. discloses a cooling assembly for fuel cells whereby working fluid is circulated through a conduit arranged in serpentine fashion within a member of the cooling assembly. The conduit can be constructed as a single, manifold-free, continuous working fluid passage means having only one inlet and one outlet. Kaufman et al. disclose no means to equalize working fluid flow through multiple conduits by varying in resistance along the flow path of the working fluid.
U.S. Pat. No. 5,085,058 to Aaron et al. discloses a bi-flow expansion device for a heat pump or other apparatus where fluid travel is reversed with different required flow rates in each direction to obviate the need for two expansion devices. This device is used in a single flow line with a large pressure drop and does not equalize working fluid flow through multiple condensation tubes.
The prior art disclosures discussed above, where applicable, provides no more than variations of the generally known techniques for equalizing flow in parallel condensation tubes that require subcooling of the working fluid. Thus, a need exists for additional useful flow equalization mechanisms, particularly for a space-based cooling systems whose requirements severely restrict weight, space, and power allowances. Those skilled in the art will appreciate the novel features of the present invention that solves these and other problems.